1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets in various image processing apparatuses, such as a printer, a copier and a document reading apparatus.
2. Description of the Related Art
In general, a sheet feeding apparatus includes a document feeding apparatus for feeding a document having information recorded therein placed on a document tray to a document mount (contact glass) for reading image information and a paper feeding apparatus for feeding it to a printing portion for printing read image information on recording paper. These apparatuses feed a bundle of documents or paper sheet by sheet to the document mount or the printing portion.
When feeding a sheet such as the document or the recording paper, there are the cases where multiple sheets are overlappingly fed due to static electricity, humidity or the like. For that reason, there is a problem that the fed sheets lag in a feeding path and a paper jam occurs.
Thus, the sheet feeding apparatus is provided with an overlapped feeding detection apparatus for detecting whether the sheets are fed one by one. For instance, in Japanese Patent Laid-Open No. 60-178141 and Japanese Patent Laid-Open No. 04-197946, transit time of the fed sheet from its anterior end to its posterior end is detected by using a sheet passage detection apparatus for detecting whether the sheet has passed a paper path, and a length of the passed sheet is determined from the detected transit time so as to determine whether there is overlapped feeding by whether the length is longer than a specified sheet length.
Japanese Patent Laid-Open No. 09-235033 provides an ultrasonic sensor for generating an ultrasonic wave from a wave transmitter and receiving the ultrasonic wave with a wave receiver, where the ultrasonic wave is applied to the sheet being fed and the ultrasonic wave having passed the sheet is received by the wave receiver so as to determine whether there is the overlapped feeding from a change in a ultrasonic signal volume received.
In the cases of Japanese Patent Laid-Open No. 60-178141 and Japanese Patent Laid-Open No. 04-197946, it is determined whether there is the overlapped feeding from the length of the sheet having passed the paper path. Therefore, the overlapped feeding cannot be detected in the case where the feeding is performed with the sheets overlapping due to static electricity or the like, that is, in an overlapping state with no displacement between the sheets.
In the case of Japanese Patent Laid-Open No. 09-235033, it is determined whether there is the overlapped feeding by applying the ultrasonic wave to the sheet being fed, that is, a moving sheet. For that reason, an ultrasonic signal received on the wave receiver side fluctuates due to undulation of the sheet or the like so that the overlapped feeding cannot be accurately detected.
Thus, in view of the problems, an object of the present invention is to provide a sheet feeding apparatus including an overlapped feeding detection apparatus which can securely detect the overlapped feeding as to the sheet being fed, and a document feeding apparatus and an image processing apparatus including the sheet feeding apparatus.